This invention relates to a novel precast concrete board for use as the panel or floor tile. More particularly, this invention relates to a novel precast concrete board which has a dense pore-free surface having the appearance of porcelain tile or hewn natural stone and which is provided with better durability, improved adiabatic property and fire resistance, together with water immpermeability and a process for producing the same utilizing industrial wastes and new resources which have never hitherto been utilized.
For the exterior finish in construction works, porcelain tiles and natural stone materials are used. Sometimes sculpture or relief work is applied in situ. Recently, precast concrete boards are widely used. Conventional precast concrete boards for exterior finish have a decorative surface formed by spraying of colored mortar, for instance, and they are superior to the conventional porcelain tiles that are produced by ceramic art, in which much energy is required, and products of larger size cannot be obtained, and they must be applied to the walls by plastering. However, precast concrete boards are inferior to the porcelain tiles in decorative nature and are defective in that efflorescence occurs on their surface.
The known precast concrete boards are made by casting concrete into a knockdown mold made of metal plates to the inside of which a mold release agent has been applied. In this case, through the chinks or the narrow openings between the knockdown mold frames, water flows out and this allows water to diffuse and exude from inside of the concrete mortar that is setting and hardening to the surface thereof, and this causes formation of surface pores or pin holes, and these make the boards water-permeable and cause efflorescence later.
Most of the known precast concrete boards are solid. Therefore, for production thereof, a large amount of precious natural resources such as cement, sand and gravel are required, and the boards themselves are very heavy. The heavy solid boards are not easy to handle, are easily broken when exposed to fire, and are inferior in adiabatic effect.
This invention provides a novel precast concrete board with decorative surface and light porous core or lining in which the above mentioned defects are diminished and relates to a process for production thereof utilizing natural resources which hitherto have not been utilized, too.
Prior to this invention of mine varieties of materials of this kind were known. For instance, in U.S. Pat. No. 1,702,730, a laminated wall product comprising in combination two dense hard facings and an intermediate core of cellular texture is disclosed. This is a gypsum board, and therefore inferior in the decorative effect. The cellular structure of the core layer is formed by effervescence in the reaction of the added acid (sulfuric acid) and carbonates contained in gypsum as impurities. When the carbonate content is insufficient, carbonates are added. Therefore quality control in the production thereof is extremely difficult and use of such a strong acid is not desirable.
In U.S. Pat. No. 2,021,359, a sound absorbing wall material is disclosed, which comprises a pad of felted fibers coated by water-proofing film, said fibers being confined in walls of cementinous material. This wall material is intended for sound absorption and therefore inferior in fire resistance and strength.
In U.S. Pat. No. 2,152,190, a building block having a light weight cellular body and a dense strong outer layer enclosing said body. In this patent it is not mentioned how the light weight cellular core cement body is made. It is surmised that it is made by foaming or air-entraining in the cement mortar. This requires a rather large scale apparatus and quality control is extremely difficult.
As seen in the above, cellular structure of inorganic cement material was formed by chemical effervescence or physical foaming. These techniques were not satisfactory in that they require rather large scale apparatus and quality control was very difficult.
However, the deficiency of the prior art has been removed by employment of a siliceous volcanic sand.
The above-mentioned siliceous volcanic sand is the one called "shirasu" in Japan that is found in abundance in volcanic zones in Japan, Taiwan and some other Asian countries. Although little is known about the origine thereof, it is believed that "shirasu" includes both flow-out and fall-out. It is found in the form of easily degraded sand mass. It is siliceous and therefore has good affinity to alkaline material. Recently it has been found that when it is heated, the sand particles swell and become minute balloons. I found that the expanded and non-expanded "shirasu" are inexpensive excellent aggregate and filler for the concrete board of this invention.